Thermodynamic Modeling of Uranium and Oxygen Containing Ternary Systems with Gadolinium, Lanthanum, and Thorium

The CALPHAD method is used to assess the thermodynamic properties and phase relations in the U-M-O system where M = Gd, La, and Th. A compound energy formalism (CEF) model for fluorite UO_2±x [urania] is extended to represent the complex U_1-yM_yO_2±x [urania solid solution] phases. The lattice stabilities for fictive GdO₂ [gadolinia] and LaO₂ [lanthana] fluorite structure compounds are calculated from density functional theory (DFT) for use in the CEF for U_1-yM_yO_2±x [urania solid solution phase] while U⁶⁺ [uranium 6 plus cation] is introduced into the cation sublattice of the CEF for U_1-yM_yO_2±x [urania solid solution phase] to better reproduce phase relations in U-Ln-O systems at high fixed trivalent Ln [lanthanide] compositions. Tentative Gibbs functions and CEF representations for the fluorite derivative rhombohedral phases were developed and the two-sublattice liquid model (TSLM) was used to describe the melt.

Equilibrium oxygen pressures over U_1-yTh_yO_2±x [urania thoria solid solution] were obtained from thermogravimetric measurements and used together with those reported in the literature, phase relations, and other experimentally determined thermodynamic values to fit adjustable parameters of the CEF and TSLM along with the standard state enthalpy and entropy of the Gibbs functions representing the stoichiometric compounds. The models can be extended to include other actinides and fission products to develop higher order multi-component system assessments to support further experimental efforts and the development of multi-physics fuel performance simulation codes.